EMMA ALBERICI, PRESENTER: The conclave to elect a new pope is about to get underway at the Vatican.

The cardinals have held a mass in St Peter's Basilica and will hold the first vote overnight.

The new pope will take office against the backdrop of clergy sexual abuse and allegations of corruption at the Vatican bank. He'll also have to reconcile the declining support in the West with the spread of Catholicism in the developing world.

Europe correspondent Philip Williams reports from the Vatican City.

PHILIP WILLIAMS, REPORTER: Inside St Peter's Basilica, the weight of expectation hung heavily on the cardinals. Last prayers before they were being shut away from the world.

There may be 1.2 billion Catholics with an interest in the outcome, but only 115 cardinals will decide.

There was a spontaneous outpouring of appreciation for the outgoing pontiff, Benedict. And questions in everyone's mind: who amongst these men will be next?

Speculation in the Italian media has fallen into two camps. Those who want to reform the Vatican bureaucracy, known as the Curia, favour the Italian cardinal Angelo Scola. The Milan bishop is receiving support from the Americans who believe the 71-year-old has the clout and experience as an administrator to carry out reform.

On the other side is a group of Vatican insiders resisting internal change. They appear to be supporting Brazilian Odilo Scherer. He heads the popular Sao Paulo diocese but spent seven years in the Vatican working at the important Congregation of Bishops. Cardinal Scherer believes in engaging with the media and has a constant presence on Twitter.

ODILO SCHERER, ARCHBISHOP OF SAO PAULO (Feb. 13, voiceover translation): What we have to face right now is the coming together of consequences of a culture that has expanded very quickly in light of the new conditions for communication and the spreading of all forms of cultural expression. And this is a very big challenge we are facing right now.

PHILIP WILLIAMS: A potential compromise candidate is the American Sean O'Malley. The 68-year-old topped an expert's poll in Italy's biggest selling newspaper and was also the reader's leading choice.

SEAN O'MALLEY, ARCHBISHOP OF BOSTON: Let's pray that the Holy Spirit (inaudible) in the Church to choose a new pope who will confirm us in our faith and make more visible the (inaudible).

PHILIP WILLIAMS: As a Franciscan he's seen as being close to the poor. Many also view him as strong on clergy sex abuse, having instituted what he termed a policy of zero tolerance in Boston, although some victims groups say he hasn't done enough. Another factor going against Cardinal O'Malley is his American citizenship and concerns about appointing a superpower pope.

That could leave Canada's Marc Ouellet as the strongest North American contender. The 68-year-old heads the congregation for bishops and his thinking is closely linked to that of the last pope. He might also be helped by his close connections with the Church in Latin America.

If the cardinals look to the developing world, the next pope could be Ghana's Peter Turkson. The 64-year-old heads the Vatican's Council for Peace and Justice. A critic of unchecked capitalism, he expressed support for the Occupy Wall Street movement, but is staunchly conservative on social issues.

So, how long before we know who'll lead the world's 1.2 billion Catholics?

Let's start by looking at the meaning of the word conclave. In Latin it literally means "with key". The term dates back to 1271 when cardinals couldn't agree on a new pope for two and a half years. To break the deadlock they were locked up and given only bread and water until they made a decision.

Well these days, it's not that extreme. Cardinals vote, sleep and eat within closed areas. No TVs, radios, newspapers and no mobile phones, so don't expect a tweet. After a mass on the first day of the conclave, the doors are locked. Cardinals have the option of holding one ballot. It's unlikely a new pope will emerge on day one.

From then on, cardinals hold four ballots each day; two in the morning and two in the evening. All ballot papers are secret and they're burned to each count. A special dye ensures the smoke is black when there's no decision and white when the new pope is finally elected.

Between now and the puff of white smoke will be a great deal of praying and some old-fashioned power politics.